There have been available over the years many apparatuses for measuring opacity or transmissivity of materials. Such apparatuses have proven useful in production areas where it is necessary to monitor not only the presence or absence of objects, but further to more accurately measure aspects of a material relating to its opacity or transmissivity. For example, an apparatus that measures opacity of a material can be used for determining the thickness of such material, which information can be used for quality control.
Still further, an opacity measurement apparatus is provided for determining a density of an effluent flowing in a furnace stack as described in U.S. Pat. No. 4,076,425, issued to Saltz. This apparatus uses a first light path directed through the effluent and a second light path external to the effluent. A ratio is determined from measurements taken from the first and second light paths for indicating the opacity of the effluent. Such apparatuses as described by Saltz, however, include several reflection mirrors and lenses and can be relatively cumbersome, complex, and sensitive.
Suzuki et al., in U.S. Pat. No. 5,231,576, teaches an apparatus for testing specimen concentrations (blood) by reflecting a light off of the specimen, which apparatus requires a 60 second delay after the switch 36 is enabled by loading a test piece. Brunnschweiler, in published patent application GB 21772102A, teaches a light transmission measuring system using a second detector 14a for controlling the intensity of the light source.
Automobile window tinting presents a field where it has become necessary to determine the transmittance property of the tinted glass. Such transmittance measurements are made ever more urgent because several states have passed legislation regulating the allowable transmittance of automobile windows. In some localities, enforcement agencies must resort to subjective tests in determining whether a given window is tinted too dark. For example, one known method of checking a window tint is accomplished by viewing a specially marked card through the window while making a visual check of visible patterns printed on the card. Enforcing such recently enacted legislation, however, requires a more scientific, accurate and repeatable measurement. For practicality purposes, this requires a portable, battery operated, transmittance meter having accurate measurement capabilities. Such a meter must not only be rugged, but also able to maintain accuracy so that any measurements taken will meet minimum evidentiary requirements in court.
One such attempt of providing a portable window transmittance meter, for use by law-enforcement personnel, for example, is described in U.S. Pat. No. 5,073,707 issued to Marcin. The apparatus described therein includes a housing having a slot which may be slipped over a window pane. A switch detects the initial entry of the window pane into the slot which almost immediately thereafter causes a light emitting diode to transmit light that is measured and stored. Since the window pane has not yet fully entered the slot, the transmittance of air is initially measured for use as a reference. A second light measurement is made approximately three seconds later when the window pane is assumed to be fully inserted. A ratio is determined between the first reference measurement and the second measurement for providing an indication of the window's transmittance.
Notwithstanding Saltz and Marcin, there still exists a need to improve both the reliability and accuracy of the transmittance meter. Specifically, Saltz's apparatus is inappropriate for portable use, and Marcin's meter may be affected by stray light, and may further produce false readings due to improper use, for example, from light scattering when the window is not properly inserted or when inserted too slowly (or moved after insertion but before the second measurement). The light scattering errors can occur from light scattered off the edge of the glass. Stray light should be accounted for and substantially eliminated for highly accurate measurements. Still further, inaccuracies may occur due to environmental conditions and circuit tolerances.
These problems are further exacerbated by the need to test those windows that are not accessible for sliding a portable window transmittance meter thereover. For example, a stationary back window or a rear window may be tinted at a different darkness level than the other movable windows. None of the heretofore known portable meters provide a capability of placing a transmitter and detector on opposing sides of such a stationary window. Other problems present themselves in such a situation, for example, even if transmitters and receivers are so placed, they must be accurately aligned, a task that can be made especially difficult given limited arm lengths.
Thus, what is needed is a portable window transmittance meter having a capability to place a transmitter and receiver on opposing sides of a substrate not having an open edge available, the transmitter and receiver being capable of self alignment so as not to depend upon an accuracy of manual alignment accuracy for such tests.